Well, congrats on having set some big goals for yourself. It never hurts to aim high :)
First, the easy one. Depending on where you live, owning your own company simply costs a little money to start and a little more money+time to keep going. E.g. in the UK it cost me £35 to have a company do it all for me. I just told them the company name and my details and they did the rest. Every year I have to file some company accounts and pay a few fees and that's it. It helps that I don't have any employees, I'm just the unpaid Director ;)
Next up, all the rest of your goals - this is a little trickier...
The game side of things: There are several areas involved in making games, which you'll be learning as you go along:
- Game Theory - making a good, fun game. Try reading "The Art of Game Design: A book of lenses" by Jesse Schell, or other books on the general theory of game design.
- Software Development - The nuts and bolts of getting the game working. Either build everything from scratch or speed things up by using existing engines.
- Graphics - Making it look pretty. Get an artist or graphic designer on board, people can usually spot Developer Art a mile off.
- Sound - Never underestimate the power of good sound effects and music.
- Marketing - I started thinking about this as I released the game, which was way too late. You're young so hopefully social media and constantly blogging everything comes easier to you than me :D
There's all sorts of other things, but those are the broad strokes. The first thing to remember is that you're going to make mistakes in all the above areas (don't worry, everyone does). Then you'll figure out what you did wrong and do it better next time, and if you repeat that often enough you'll become good at it and a professional in that area.
What does this have to do with your goals? Simply that you need to build your teams skills and abilities, and keep their motivation for doing more projects. If you start a huge project that'll take you all 2 years (at best) with nothing to show until the end, while none of you know quite what / how to do it, then I'm afraid your chances of finishing aren't so good. If this is the first project you're all working together on then I suggest trying something a little more achievable than a full scale 3D MMO based off your own game engine.
Try building up to your planned big game with a series of smaller apps and games that help your team learn what they need to know. E.g.
- Simple app or browser based single player game - not MMO :) - Simple game play, simple (2D) graphics. Gives you all experience in creating all aspects of a game, should shake out any problems in the team and get you all used to the process. Plus it shouldn't take a team too long at all (3-6 months?), so you'll all have something to show for your efforts and can feel good about doing it. If you want to create games, use an existing game engine to speed things up. Otherwise factor in the extra time needed to create your own framework and get the graphics guys working on even better art work with all their spare time.
- 3D app or browser single player game - Same as number 1 above, add 3D and some other features that you want your main game to have. 3D adds a lot of graphical overheads, plus requires a different display engine. Use an existing game engine to speed up development if you want to make the actual game, or continue developing and improving your game engine from the first game.
- Multiplayer game - convert one of your games to support multiplayer gameplay, or write a new one. If you plan this in from the start it can be pretty easy, otherwise it can be a big re-working. Either way, you'll learn a lot about what's needed in this area, plus you get to re-use your projects.
- Stop and realise you currently have a portfolio of released games people can play, which is awesome.
- Make your big game. By now you've got most of the experience you need to create the game you want. Sure, there's more to learn but now it's a step up to it instead of a giant leap.
Add more games in there if you like. You'll know more about what you need to learn once you've actually started, and may find you can jump to your big idea right after your first game.
One note: I say focus on creating and releasing your games for a reason (which some may disagree with)... It's easy to cut corners on quality in pet projects that aren't for release ("it crashes if you open that door, but that's ok", "Ignore that texture, it's just filler graphics", etc). By committing to creating a finished product right from the start it changes the level of quality you're aiming for (or it should, at least), and gives you a different mindset when thinking about things. And by releasing your game you get real feedback on how well / badly you've done so you can learn from the experience.
If you follow and commit to this sort of process - progressively improving your skills and producing demonstrable evidence of your achievements - then becoming a professional <anything> should pretty much take care of itself.
Anyway, hope some of that helps you all.
Good luck!
